Really difficult OSPF question to routing gurus

Discussion in 'Cisco' started by Oleg Tipisov, Aug 18, 2004.

  1. Oleg Tipisov

    Oleg Tipisov Guest

    Hi!

    What is the difference between E1 and E2 external LSA types (read on!)
    ?

    In production we have the following network:

    R1 ========= R2

    i.e. two routers connected by two links: GRE tunnel and E1. The
    bandwidth is set to 1500 on the tunnel interface and to 2000 on the
    serial. Both links are in the same area 172. R2 (ASBR) announces
    external (type 2) route 10.2.0.0/16 to R1 via both links with the same
    metric 20.

    R1# sh ip ospf database external 10.2.0.0

    OSPF Router with ID (172.16.31.1) (Process ID 1)

    Type-5 AS External Link States

    Routing Bit Set on this LSA
    LS age: 1513
    Options: (No TOS-capability, DC)
    LS Type: AS External Link
    Link State ID: 10.2.0.0 (External Network Number )
    Advertising Router: 1.1.1.2
    LS Seq Number: 8000292C
    Checksum: 0x57E3
    Length: 36
    Network Mask: /16
    Metric Type: 2 (Larger than any link state path)
    TOS: 0
    Metric: 20
    Forward Address: 0.0.0.0
    External Route Tag: 0

    The OSPF cost of R1 interfaces is *not* added to the metric, as the
    LSA type is 2. In the routing table we see the following:

    R1# sh ip route 10.2.0.0
    Routing entry for 10.2.0.0/16
    Known via "ospf 1", distance 110, metric 20, type extern 2, forward
    metric 61
    Last update from 172.16.30.6 on Serial0/0:1, 00:32:13 ago
    Routing Descriptor Blocks:
    * 172.16.30.6, from 1.1.1.2, 00:32:13 ago, via Serial0/0:1
    Route metric is 20, traffic share count is 1

    As you can see the traffic is *not* load-balanced across the two links
    despite of the fact that the metric should be the same via both links
    (20).

    First (simple) question: Why?

    Second (difficult) question: In this case what is the difference
    between E1 and E2 LSA types? I mean they both give us the same result
    here. Can anybody create an example where they result in two
    *different* routing tables?

    Thx,
    Oleg Tipisov,
    REDCENTER
    Oleg Tipisov, Aug 18, 2004
    #1
    1. Advertising

  2. Oleg Tipisov <> wrote:
    > Hi!
    >
    > What is the difference between E1 and E2 external LSA types (read on!)
    > ?
    >
    > In production we have the following network:
    >
    > R1 ========= R2
    >
    > i.e. two routers connected by two links: GRE tunnel and E1. The
    > bandwidth is set to 1500 on the tunnel interface and to 2000 on the
    > serial. Both links are in the same area 172. R2 (ASBR) announces
    > external (type 2) route 10.2.0.0/16 to R1 via both links with the same
    > metric 20.
    >
    > R1# sh ip ospf database external 10.2.0.0
    >
    > OSPF Router with ID (172.16.31.1) (Process ID 1)
    >
    > Type-5 AS External Link States
    >
    > Routing Bit Set on this LSA
    > LS age: 1513
    > Options: (No TOS-capability, DC)
    > LS Type: AS External Link
    > Link State ID: 10.2.0.0 (External Network Number )
    > Advertising Router: 1.1.1.2
    > LS Seq Number: 8000292C
    > Checksum: 0x57E3
    > Length: 36
    > Network Mask: /16
    > Metric Type: 2 (Larger than any link state path)
    > TOS: 0
    > Metric: 20
    > Forward Address: 0.0.0.0
    > External Route Tag: 0
    >
    > The OSPF cost of R1 interfaces is *not* added to the metric, as the
    > LSA type is 2. In the routing table we see the following:
    >
    > R1# sh ip route 10.2.0.0
    > Routing entry for 10.2.0.0/16
    > Known via "ospf 1", distance 110, metric 20, type extern 2, forward
    > metric 61
    > Last update from 172.16.30.6 on Serial0/0:1, 00:32:13 ago
    > Routing Descriptor Blocks:
    > * 172.16.30.6, from 1.1.1.2, 00:32:13 ago, via Serial0/0:1
    > Route metric is 20, traffic share count is 1
    >
    > As you can see the traffic is *not* load-balanced across the two links
    > despite of the fact that the metric should be the same via both links
    > (20).
    >
    > First (simple) question: Why?
    >


    RFC1583
    16.8. Equal-cost multipath
    The OSPF protocol maintains multiple equal-cost routes to all destinations.
    This can be seen in the steps used above to calculate the routing table,
    and in the definition of the routing table structure.

    Each one of the multiple routes will be of the same type (intra-area,
    inter-area, type 1 external or type 2 external), cost, and will have the
    same associated area. However, each route specifies a separate next hop
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    and Advertising router.
    ^^^^^^^^^^^^^^^^^^^^^^

    I don't understand the second question (It must be too difficult :))

    --
    dg
    Daniel Ginsburg, Aug 18, 2004
    #2
    1. Advertising

  3. Oleg Tipisov

    Ivan Ostres Guest

    In article <>,
    says...
    > Hi!
    >
    > What is the difference between E1 and E2 external LSA types (read on!)
    > ?
    >
    > In production we have the following network:
    >
    > R1 ========= R2
    >
    > i.e. two routers connected by two links: GRE tunnel and E1. The
    > bandwidth is set to 1500 on the tunnel interface and to 2000 on the
    > serial. Both links are in the same area 172. R2 (ASBR) announces
    > external (type 2) route 10.2.0.0/16 to R1 via both links with the same
    > metric 20.
    >
    > R1# sh ip ospf database external 10.2.0.0
    >
    > OSPF Router with ID (172.16.31.1) (Process ID 1)
    >
    > Type-5 AS External Link States
    >
    > Routing Bit Set on this LSA
    > LS age: 1513
    > Options: (No TOS-capability, DC)
    > LS Type: AS External Link
    > Link State ID: 10.2.0.0 (External Network Number )
    > Advertising Router: 1.1.1.2
    > LS Seq Number: 8000292C
    > Checksum: 0x57E3
    > Length: 36
    > Network Mask: /16
    > Metric Type: 2 (Larger than any link state path)
    > TOS: 0
    > Metric: 20
    > Forward Address: 0.0.0.0
    > External Route Tag: 0
    >
    > The OSPF cost of R1 interfaces is *not* added to the metric, as the
    > LSA type is 2. In the routing table we see the following:
    >
    > R1# sh ip route 10.2.0.0
    > Routing entry for 10.2.0.0/16
    > Known via "ospf 1", distance 110, metric 20, type extern 2, forward
    > metric 61
    > Last update from 172.16.30.6 on Serial0/0:1, 00:32:13 ago
    > Routing Descriptor Blocks:
    > * 172.16.30.6, from 1.1.1.2, 00:32:13 ago, via Serial0/0:1
    > Route metric is 20, traffic share count is 1
    >
    > As you can see the traffic is *not* load-balanced across the two links


    Oleg, there's not enough data to troubleshoot this. It would be nice to
    see 'sh ip route' from R1 and 'show ip ospf database' from R1.


    --
    -Ivan.

    *** Use Rot13 to see my eMail address ***
    Ivan Ostres, Aug 19, 2004
    #3
  4. Oleg Tipisov

    JNCIP#0136 Guest

    Hello,
    A1. By default, GRE tunnel has OSPF cost 11111 and E1 link has OSPF cost 50.
    The forwarding address of Your External LSA is 0.0.0.0 (meaning the ASBR
    itself, i.e. ASBR router-id) and least coast path to ASBR is via E1. See
    RFC2328 section 16.4 (3) for AS external path selection rules.
    A2. You will definitely see a difference in routing table if You manage to
    get ASBR to advertise 10.2/16 in two AS external LSAs (one type 1 and second
    type 2). If You simply redistribute once 10.2/16 (as static or connected)
    into OSPF and specify "type" as 1 or 2 that will give You only one AS
    external LSA.
    HTH,
    Cheers
    Alex


    "Oleg Tipisov" <> wrote in message
    news:...
    > Hi!
    >
    > What is the difference between E1 and E2 external LSA types (read on!)
    > ?
    >
    > In production we have the following network:
    >
    > R1 ========= R2
    >
    > i.e. two routers connected by two links: GRE tunnel and E1. The
    > bandwidth is set to 1500 on the tunnel interface and to 2000 on the
    > serial. Both links are in the same area 172. R2 (ASBR) announces
    > external (type 2) route 10.2.0.0/16 to R1 via both links with the same
    > metric 20.
    >
    > R1# sh ip ospf database external 10.2.0.0
    >
    > OSPF Router with ID (172.16.31.1) (Process ID 1)
    >
    > Type-5 AS External Link States
    >
    > Routing Bit Set on this LSA
    > LS age: 1513
    > Options: (No TOS-capability, DC)
    > LS Type: AS External Link
    > Link State ID: 10.2.0.0 (External Network Number )
    > Advertising Router: 1.1.1.2
    > LS Seq Number: 8000292C
    > Checksum: 0x57E3
    > Length: 36
    > Network Mask: /16
    > Metric Type: 2 (Larger than any link state path)
    > TOS: 0
    > Metric: 20
    > Forward Address: 0.0.0.0
    > External Route Tag: 0
    >
    > The OSPF cost of R1 interfaces is *not* added to the metric, as the
    > LSA type is 2. In the routing table we see the following:
    >
    > R1# sh ip route 10.2.0.0
    > Routing entry for 10.2.0.0/16
    > Known via "ospf 1", distance 110, metric 20, type extern 2, forward
    > metric 61
    > Last update from 172.16.30.6 on Serial0/0:1, 00:32:13 ago
    > Routing Descriptor Blocks:
    > * 172.16.30.6, from 1.1.1.2, 00:32:13 ago, via Serial0/0:1
    > Route metric is 20, traffic share count is 1
    >
    > As you can see the traffic is *not* load-balanced across the two links
    > despite of the fact that the metric should be the same via both links
    > (20).
    >
    > First (simple) question: Why?
    >
    > Second (difficult) question: In this case what is the difference
    > between E1 and E2 LSA types? I mean they both give us the same result
    > here. Can anybody create an example where they result in two
    > *different* routing tables?
    >
    > Thx,
    > Oleg Tipisov,
    > REDCENTER
    JNCIP#0136, Aug 20, 2004
    #4
  5. Oleg Tipisov

    carl Guest

    Hi

    I guess the

    < forward metric 61> in the output of 'show ip route 10.2.0.0' was the
    deciding factor BUT how does this make Type E1 and Type E2 different if the
    ASBR has multiple links to a router? The outcome from both is the same??

    Carl

    "JNCIP#0136" <> wrote in message
    news:cg4cig$pul$...
    > Hello,
    > A1. By default, GRE tunnel has OSPF cost 11111 and E1 link has OSPF cost

    50.
    > The forwarding address of Your External LSA is 0.0.0.0 (meaning the ASBR
    > itself, i.e. ASBR router-id) and least coast path to ASBR is via E1. See
    > RFC2328 section 16.4 (3) for AS external path selection rules.
    > A2. You will definitely see a difference in routing table if You manage to
    > get ASBR to advertise 10.2/16 in two AS external LSAs (one type 1 and

    second
    > type 2). If You simply redistribute once 10.2/16 (as static or connected)
    > into OSPF and specify "type" as 1 or 2 that will give You only one AS
    > external LSA.
    > HTH,
    > Cheers
    > Alex
    >
    >
    > "Oleg Tipisov" <> wrote in message
    > news:...
    > > Hi!
    > >
    > > What is the difference between E1 and E2 external LSA types (read on!)
    > > ?
    > >
    > > In production we have the following network:
    > >
    > > R1 ========= R2
    > >
    > > i.e. two routers connected by two links: GRE tunnel and E1. The
    > > bandwidth is set to 1500 on the tunnel interface and to 2000 on the
    > > serial. Both links are in the same area 172. R2 (ASBR) announces
    > > external (type 2) route 10.2.0.0/16 to R1 via both links with the same
    > > metric 20.
    > >
    > > R1# sh ip ospf database external 10.2.0.0
    > >
    > > OSPF Router with ID (172.16.31.1) (Process ID 1)
    > >
    > > Type-5 AS External Link States
    > >
    > > Routing Bit Set on this LSA
    > > LS age: 1513
    > > Options: (No TOS-capability, DC)
    > > LS Type: AS External Link
    > > Link State ID: 10.2.0.0 (External Network Number )
    > > Advertising Router: 1.1.1.2
    > > LS Seq Number: 8000292C
    > > Checksum: 0x57E3
    > > Length: 36
    > > Network Mask: /16
    > > Metric Type: 2 (Larger than any link state path)
    > > TOS: 0
    > > Metric: 20
    > > Forward Address: 0.0.0.0
    > > External Route Tag: 0
    > >
    > > The OSPF cost of R1 interfaces is *not* added to the metric, as the
    > > LSA type is 2. In the routing table we see the following:
    > >
    > > R1# sh ip route 10.2.0.0
    > > Routing entry for 10.2.0.0/16
    > > Known via "ospf 1", distance 110, metric 20, type extern 2, forward
    > > metric 61
    > > Last update from 172.16.30.6 on Serial0/0:1, 00:32:13 ago
    > > Routing Descriptor Blocks:
    > > * 172.16.30.6, from 1.1.1.2, 00:32:13 ago, via Serial0/0:1
    > > Route metric is 20, traffic share count is 1
    > >
    > > As you can see the traffic is *not* load-balanced across the two links
    > > despite of the fact that the metric should be the same via both links
    > > (20).
    > >
    > > First (simple) question: Why?
    > >
    > > Second (difficult) question: In this case what is the difference
    > > between E1 and E2 LSA types? I mean they both give us the same result
    > > here. Can anybody create an example where they result in two
    > > *different* routing tables?
    > >
    > > Thx,
    > > Oleg Tipisov,
    > > REDCENTER

    >
    >
    carl, Aug 21, 2004
    #5
  6. Oleg Tipisov

    carl Guest

    Hi

    I guess the

    < forward metric 61> in the output of 'show ip route 10.2.0.0' was the
    deciding factor BUT how does this make Type E1 and Type E2 different if the
    ASBR has multiple links to a router? The outcome from both is the same??

    Carl

    "JNCIP#0136" <> wrote in message
    news:cg4cig$pul$...
    > Hello,
    > A1. By default, GRE tunnel has OSPF cost 11111 and E1 link has OSPF cost

    50.
    > The forwarding address of Your External LSA is 0.0.0.0 (meaning the ASBR
    > itself, i.e. ASBR router-id) and least coast path to ASBR is via E1. See
    > RFC2328 section 16.4 (3) for AS external path selection rules.
    > A2. You will definitely see a difference in routing table if You manage to
    > get ASBR to advertise 10.2/16 in two AS external LSAs (one type 1 and

    second
    > type 2). If You simply redistribute once 10.2/16 (as static or connected)
    > into OSPF and specify "type" as 1 or 2 that will give You only one AS
    > external LSA.
    > HTH,
    > Cheers
    > Alex
    >
    >
    > "Oleg Tipisov" <> wrote in message
    > news:...
    > > Hi!
    > >
    > > What is the difference between E1 and E2 external LSA types (read on!)
    > > ?
    > >
    > > In production we have the following network:
    > >
    > > R1 ========= R2
    > >
    > > i.e. two routers connected by two links: GRE tunnel and E1. The
    > > bandwidth is set to 1500 on the tunnel interface and to 2000 on the
    > > serial. Both links are in the same area 172. R2 (ASBR) announces
    > > external (type 2) route 10.2.0.0/16 to R1 via both links with the same
    > > metric 20.
    > >
    > > R1# sh ip ospf database external 10.2.0.0
    > >
    > > OSPF Router with ID (172.16.31.1) (Process ID 1)
    > >
    > > Type-5 AS External Link States
    > >
    > > Routing Bit Set on this LSA
    > > LS age: 1513
    > > Options: (No TOS-capability, DC)
    > > LS Type: AS External Link
    > > Link State ID: 10.2.0.0 (External Network Number )
    > > Advertising Router: 1.1.1.2
    > > LS Seq Number: 8000292C
    > > Checksum: 0x57E3
    > > Length: 36
    > > Network Mask: /16
    > > Metric Type: 2 (Larger than any link state path)
    > > TOS: 0
    > > Metric: 20
    > > Forward Address: 0.0.0.0
    > > External Route Tag: 0
    > >
    > > The OSPF cost of R1 interfaces is *not* added to the metric, as the
    > > LSA type is 2. In the routing table we see the following:
    > >
    > > R1# sh ip route 10.2.0.0
    > > Routing entry for 10.2.0.0/16
    > > Known via "ospf 1", distance 110, metric 20, type extern 2, forward
    > > metric 61
    > > Last update from 172.16.30.6 on Serial0/0:1, 00:32:13 ago
    > > Routing Descriptor Blocks:
    > > * 172.16.30.6, from 1.1.1.2, 00:32:13 ago, via Serial0/0:1
    > > Route metric is 20, traffic share count is 1
    > >
    > > As you can see the traffic is *not* load-balanced across the two links
    > > despite of the fact that the metric should be the same via both links
    > > (20).
    > >
    > > First (simple) question: Why?
    > >
    > > Second (difficult) question: In this case what is the difference
    > > between E1 and E2 LSA types? I mean they both give us the same result
    > > here. Can anybody create an example where they result in two
    > > *different* routing tables?
    > >
    > > Thx,
    > > Oleg Tipisov,
    > > REDCENTER

    >
    >
    carl, Aug 21, 2004
    #6
  7. Oleg Tipisov

    carl Guest

    Hi

    forget the last post, I think I get it now. It's just the setup used in the
    example Oleg used that results in the distance to the forwarding address
    being used. And reading the RFC section 16.4 I now see the different process
    used for selecting routes when either Type E1 or Type E2.

    I guess in this particular setup if you want to use both links to forward
    traffic then you need to set the OSPF interface cost on R1 for both links to
    R2 to be the same.

    Regards

    Carl

    "carl" <> wrote in message
    news:4127973d$0$13177$...
    > Hi
    >
    > I guess the
    >
    > < forward metric 61> in the output of 'show ip route 10.2.0.0' was the
    > deciding factor BUT how does this make Type E1 and Type E2 different if

    the
    > ASBR has multiple links to a router? The outcome from both is the same??
    >
    > Carl
    >
    > "JNCIP#0136" <> wrote in message
    > news:cg4cig$pul$...
    > > Hello,
    > > A1. By default, GRE tunnel has OSPF cost 11111 and E1 link has OSPF cost

    > 50.
    > > The forwarding address of Your External LSA is 0.0.0.0 (meaning the ASBR
    > > itself, i.e. ASBR router-id) and least coast path to ASBR is via E1. See
    > > RFC2328 section 16.4 (3) for AS external path selection rules.
    > > A2. You will definitely see a difference in routing table if You manage

    to
    > > get ASBR to advertise 10.2/16 in two AS external LSAs (one type 1 and

    > second
    > > type 2). If You simply redistribute once 10.2/16 (as static or

    connected)
    > > into OSPF and specify "type" as 1 or 2 that will give You only one AS
    > > external LSA.
    > > HTH,
    > > Cheers
    > > Alex
    > >
    > >
    > > "Oleg Tipisov" <> wrote in message
    > > news:...
    > > > Hi!
    > > >
    > > > What is the difference between E1 and E2 external LSA types (read on!)
    > > > ?
    > > >
    > > > In production we have the following network:
    > > >
    > > > R1 ========= R2
    > > >
    > > > i.e. two routers connected by two links: GRE tunnel and E1. The
    > > > bandwidth is set to 1500 on the tunnel interface and to 2000 on the
    > > > serial. Both links are in the same area 172. R2 (ASBR) announces
    > > > external (type 2) route 10.2.0.0/16 to R1 via both links with the same
    > > > metric 20.
    > > >
    > > > R1# sh ip ospf database external 10.2.0.0
    > > >
    > > > OSPF Router with ID (172.16.31.1) (Process ID 1)
    > > >
    > > > Type-5 AS External Link States
    > > >
    > > > Routing Bit Set on this LSA
    > > > LS age: 1513
    > > > Options: (No TOS-capability, DC)
    > > > LS Type: AS External Link
    > > > Link State ID: 10.2.0.0 (External Network Number )
    > > > Advertising Router: 1.1.1.2
    > > > LS Seq Number: 8000292C
    > > > Checksum: 0x57E3
    > > > Length: 36
    > > > Network Mask: /16
    > > > Metric Type: 2 (Larger than any link state path)
    > > > TOS: 0
    > > > Metric: 20
    > > > Forward Address: 0.0.0.0
    > > > External Route Tag: 0
    > > >
    > > > The OSPF cost of R1 interfaces is *not* added to the metric, as the
    > > > LSA type is 2. In the routing table we see the following:
    > > >
    > > > R1# sh ip route 10.2.0.0
    > > > Routing entry for 10.2.0.0/16
    > > > Known via "ospf 1", distance 110, metric 20, type extern 2, forward
    > > > metric 61
    > > > Last update from 172.16.30.6 on Serial0/0:1, 00:32:13 ago
    > > > Routing Descriptor Blocks:
    > > > * 172.16.30.6, from 1.1.1.2, 00:32:13 ago, via Serial0/0:1
    > > > Route metric is 20, traffic share count is 1
    > > >
    > > > As you can see the traffic is *not* load-balanced across the two links
    > > > despite of the fact that the metric should be the same via both links
    > > > (20).
    > > >
    > > > First (simple) question: Why?
    > > >
    > > > Second (difficult) question: In this case what is the difference
    > > > between E1 and E2 LSA types? I mean they both give us the same result
    > > > here. Can anybody create an example where they result in two
    > > > *different* routing tables?
    > > >
    > > > Thx,
    > > > Oleg Tipisov,
    > > > REDCENTER

    > >
    > >

    >
    >
    carl, Aug 21, 2004
    #7
    1. Advertising

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